A common problem in directing a high energy laser beam toward an orbiting satellite and in establishing an optical communications link with a satellite arises from perturbations in the atmosphere which create phase aberrations in the transmitted signal. One technique for dealing with this problem employs a thin deformable mirror having a high density of actuators connected to shape the mirror's surface. An optical signal from the satellite, after passing through the atmosphere, is received by a wavefront sensor which "sees" the phase perturbations in the optical signal caused by the atmosphere. The sensor, in turn, controls the mirror actuators which deform the mirror to compensate for the aberrations. The reflected, corrected low intensity signal from the mirror is amplified by a Raman amplifier and returned to the satellite by a telescope. If desired, information may be encoded on the Raman wavelength (.lambda..sub.R) and a modulated signal returned.
The prior art optical link described above, including means for physically deforming a mirror, imposes a substantial time delay on the round trap radiation path. Thus, a satellite traveling at the speed required to sustain orbit will travel a substantial distance between the time a light signal leaves and is returned to it. However, it is important that the signal traverse the same atmospheric path from the satellite to the ground station as from the ground station to the satellite, in order for the aberration corrections to be most effective. One method of overcoming this problem has been to employ a beacon positioned in advance of the satellite to transmit the ground directed signal. This signal may originate at a laser within the beacon or be reflected off it from a ground-based laser. The distance between the beacon and the satellite is such that the signal returned by the ground station along the same path is intercepted by the satellite, which has moved into the position formerly occupied by the beacon. Different methods may be employed to establish the separation between the beacon and the satellite. For example, the beacon may be positioned upon a spar projecting from the satellite.
It will be apparent that, if the time delay implicit in the prior art approach described above could be eliminated, the need for much of the hardware, possibly including the displaced beacon, could be avoided. Accordingly, it is a primary object of the present invention to compensate for atmospheric phase aberrations much more rapidly than is possible with conventional adaptive optics. Other objects are to eliminate much of the complexity required in conventional adaptive optics, and to substantially reduce or eliminate the need for spaced beacons and receivers associated with the satellite. Other objects, features, and advantages will become apparent from the following description and appended claims.